1. Field of the Invention
The present invention relates to a switching power supply circuit that steps up the input voltage from a direct-current source and then feeds it to a load. More particularly, the present invention relates to a switching power supply circuit that activates/deactivates stepping-up operation according to an external signal.
2. Description of Related Art
In recent years, white light-emitting diodes that are excellent in durability, luminous efficiency, and space-saving, etc., have come to be used as one of the illumination light sources (backlights or frontlights) of liquid crystal display devices (LCDs) incorporated in electronic apparatuses such as cellular phones, PDAs (personal digital assistants), and digital cameras. This white light-emitting diode requires a relatively high forward direction voltage to emit light, and, in general, as an illumination light source, a plurality of white light-emitting diodes are used and connected in series to make their brightness equal. Thus, driving the white light-emitting diode as an illumination light source requires a direct voltage higher than the direct voltage fed from the battery incorporated in the electronic apparatus.
For this reason, a stepping-up switching power supply circuit is used as a circuit for driving the white light-emitting diode. FIG. 11A shows an example of the configuration of a conventional stepping-up switching power supply circuit. The switching power supply circuit shown in FIG. 11A is composed of an input capacitor 2, a coil 3, a diode 4 that is a rectifying device, an output capacitor 5, an output current detection resistance R1, and a stepping-up chopper regulator 10 that takes the form of a single IC package and performs stepping-up operation by controlling the charge and discharge of energy to the coil 3. The switching power supply circuit shown in FIG. 11A steps up the direct voltage fed from a direct-current source 1 such as a lithium-ion battery, then feeds the stepped-up voltage to the loads, namely, six white light-emitting diodes LED1 to LED6, and thus drives them. Note that a power source switch (not shown) is provided between the direct-current source 1 and the switching power supply circuit. When the power source switch is on, the direct voltage Vin is fed from the direct-current source 1 to the switching power supply circuit; when the power source switch is off, the direct voltage Vin is not fed from the direct-current source 1 to the switching power supply circuit.
The direct-current source 1 has the negative terminal connected to the ground, and has the positive terminal that is connected to the ground via the input capacitor 2 and is also connected to the one end of the coil 3. The other end of the coil 3 is connected to the anode of the diode 4, and the cathode of the diode 4 is connected to the ground via the output capacitor 5. The white light-emitting diodes LED1 to LED6 and the output current detection resistance R1 are connected in series to form a series circuit, and this series circuit is connected in parallel with the output capacitor 5.
The stepping-up chopper regulator 10 is provided with, as the terminals for external connection, a power supply terminal TVIN, a ground power supply terminal TGND, an output voltage monitor terminal TVO, a feedback terminal TFB, a switch terminal TVSW, and a control terminal TCTRL. The power supply terminal TVIN is connected to the positive terminal of the direct-current source 1, and the ground power supply terminal TGND is connected to the ground. With these terminals, the stepping-up chopper regulator 10 can operate from the direct-current source 1. The switch terminal TVSW is connected to the node at which the coil 3 and the diode 4 are connected together, the output voltage monitor terminal TVO is connected to the cathode of the diode 4, and the feedback terminal TFB is connected to the node at which the white light-emitting diode LED6 and the output current detection resistance R1 are connected together. The control terminal TCTRL receives a brightness adjusting signal, which will be described below.
Next, the internal configuration of the stepping-up chopper regulator 10 will be described. The stepping-up chopper regulator 10 is provided with N-channel MOSFETs 11 and 12 (hereinafter referred to as Nch transistors), a drive circuit 13, a current detection comparator 14, an oscillation circuit 15, an amplifier 16, a PWM comparator 17, an error amplifier 18, a reference power source 19, resistances R2 to R4, a soft-start circuit 20, an on/off circuit 21, an overheating detection circuit 22, an overvoltage detection circuit 23, a constant voltage circuit 24, and a switch 25.
When the switch 25 is on, the constant voltage circuit 24 converts the direct voltage Vin from the power supply terminal TVIN into a voltage having a predetermined value, and then feeds the converted voltage to the PWM comparator 17 and the error amplifier 18 as the drive voltage. When the switch 25 is on, the direct voltage Vin is fed, as the drive voltage, to the other circuits constituting the stepping-up chopper regulator 10.
The switch 25 is provided between the power supply terminal TVIN and the signal receiving side of the constant voltage circuit 24. When a high level signal is fed to the control terminal, the switch 25 is turned on; when a low level signal is fed to the control terminal, the switch 25 is turned off. The brightness adjusting signal received from the outside by the control terminal TCTRL is fed to the control terminal of the switch 25. Therefore, when the brightness adjusting signal takes a low level, electric power is not fed to the circuits of the stepping-up chopper regulator 10. This reduces the electric power consumption to near zero. This advantageously contributes to low electric power consumption.
The drains of the Nch transistor 11 and the Nch transistor 12 are both connected to the switch terminal TVSW, and the gates of the Nch transistor 11 and the Nch transistor 12 are both connected to the drive circuit 13. The source of the Nch transistor 12 is directly connected to the ground, and the source of the Nch transistor 11 is connected to the ground via the resistance R2. This makes the ratio of the drain current of the Nch transistor 11 to that of the Nch transistor 12 equal to the ratio of the gate width/gate length of the Nch transistor 11 to that of the Nch transistor 12.
The ends of the resistance R2 are respectively connected to the two input terminals of the current detection comparator 14. The output of the current detection comparator 14 and one output of the oscillation circuit 15 are added together by the amplifier 16, and their sum is fed to the inverting input terminal of the PWM comparator 17. The output of the PWM comparator 17 and the other output of the oscillation circuit 15 are fed to the drive circuit 13.
The output of the error amplifier 18 is fed to the non-inverting input terminal of the PWM comparator 17, and the non-inverting input terminal of the error amplifier 18 is connected to the feedback terminal TFB. The inverting input terminal of the error amplifier 18 is connected to one end of the resistance R3 and to one end of the resistance R4. The other end of the resistance R4 is connected to the ground, and the other end of the resistance R3 is connected to the positive terminal of the reference power source 19. The negative terminal of the reference power source 19 is connected to the ground.
The output of the soft-start circuit 20 is fed to the node at which the non-inverting input terminal of the PWM comparator 17 and the output terminal of the error amplifier 18 are connected together, and the outputs of the on/off circuit 21, the overheating detection circuit 22, and the overvoltage detection circuit 23 are fed to the drive circuit 13. The brightness adjusting signal received from the outside by the control terminal TCTRL is fed to the soft-start circuit 20 and the on/off circuit 21. The output voltage Vout is fed through the output voltage monitor terminal TVO to the overvoltage detection circuit 23.
Next, the operation of the switching power supply circuit having the configuration shown in FIG. 11A will be described. The switching power supply circuit shown in FIG. 11A causes, by making the drive circuit 13 turn on/off the Nch transistor 12, the output voltage Vout obtained by stepping up the input voltage Vin from the direct-current source 1 to appear across the output capacitor 5 and thus drives the white light-emitting diodes LED1 to LED6.
Specifically, when the drive circuit 13 applies a predetermined gate voltage to the gate of the Nch transistor 12 and the Nch transistor 12 is on, a current flows from the direct-current source 1 to the coil 3. As a result, energy is accumulated in the coil 3. On the other hand, when the drive circuit 13 does not apply a predetermined gate voltage to the gate of the Nch transistor 12 and the Nch transistor 12 is off, the accumulated energy is released, resulting in a back electromotive force generated in the coil 3. The back electromotive force generated in the coil 3 is added to the input voltage Vin of the direct-current source 1, and then charges through the diode 4 the output capacitor 5. By repeating the operations described above, stepping-up operation is performed, resulting in appearance of the output voltage Vout across the output capacitor 5. This output voltage Vout makes the output current lout flow through the white light-emitting diodes LED1 to LED6, and thereby makes the white light-emitting diodes LED1 to LED6 emit light.
Then, a feedback voltage Vfb obtained by multiplying a current value of the output current lout by a resistance value of the output current detection resistance R1 is fed through the feedback terminal TFB to the non-inverting input terminal of the error amplifier 18, and then compared with a reference voltage Vref to be fed to the inverting input terminal of the error amplifier 18. With this comparison, a voltage according to the difference between the feedback voltage Vfb and the reference voltage Vref appears in the output of the error amplifier 18. This voltage is fed to the non-inverting input terminal of the PWM comparator 17. Note that the reference voltage Vref is a voltage obtained by dividing the output voltage of the reference power source 19 by the resistances R3 and R4.
A signal proportional to the current that flows through, when the Nch transistor 11 is turned on, the resistance R2 and a sawtooth waveform signal outputted from the oscillation circuit 15 are added together and then amplified by the amplifier 16. The resultant signal is inputted to the inverting input terminal of the PWM comparator 17, and then compared with the output voltage level of the error amplifier 18 by the PWM comparator 17. As a result, when the output voltage level of the error amplifier 18 is higher than the output signal level of the amplifier 16, the PWM output of the PWM comparator 17 takes a high level. On the other hand, when the output voltage level of the error amplifier 18 is lower than the output signal level of the amplifier 16, the PWM output of the PWM comparator 17 turns to a low level.
Upon receiving the PWM output of the PWM comparator 17, the drive circuit 13 feeds a pulse signal having a duty according to the received PWM output to the gates of the Nch transistors 11 and 12 so as to turn them on/off. Specifically, when the PWM output of the PWM comparator 17 takes a high level, the drive circuit 13 starts, at the beginning of each cycle of the sawtooth waveform signal outputted from the oscillation circuit 15, feeding of the predetermined gate voltage to the Nch transistors 11 and 12 so as to turn them on. When the PWM output of the PWM comparator 17 becomes a low level, the drive circuit 13 stops feeding of the predetermined gate voltage to the Nch transistors 11 and 12 so as to turn them off.
When the above-described on/off control of the Nch transistors 11 and 12, namely, switching control operation of the drive circuit 13 is in the activated state, the stepping-up operation is activated so as to make the feedback voltage Vfb and the reference voltage Vref equal. As a result, the output current lout is stabilized to a current value obtained by dividing the reference voltage Vref (=feedback voltage Vfb) by the resistance value of the output current detection resistance R1.
The signals inputted to the inverting input terminal of the PWM comparator 17 include a signal according to the current flowing through the resistance R2, i.e., a signal according to the current flowing through the coil 3 when the Nch transistors 11 and 12 are turned on. This makes it possible to control the peak current of the coil 3. Moreover, upon detecting the output voltage Vout exceeding a predetermined voltage, the overvoltage detection circuit 23 makes the drive circuit 13 deactivate the switching control operation. This makes it possible to prevent the overvoltage exceeding the predetermined voltage from being applied to the white light-emitting diodes LED1 to LED6, which are loads, and the output capacitor 5. Furthermore, upon detecting overheating, especially around the Nch transistor 12, resulting from the switching control operation of the drive circuit 13, the overheating detection circuit 22 makes the drive circuit 13 deactivate the switching control operation. This makes it possible to prevent a breakdown or the like of the stepping-up chopper regulator 10 due to overheating.
The on/off circuit 21 makes the drive circuit 13 activates/deactivates the switching control operation according to the brightness adjusting signal inputted to the control terminal TCTRL. When the drive circuit 13 activates the switching control operation, the switching power supply circuit activates the stepping-up operation; when the drive circuit 13 deactivates the switching control operation, the switching power supply circuit deactivates the stepping-up operation. Used as the brightness adjusting signal is, for example, a PWM (pulse width modulation) signal. When the brightness adjusting signal inputted to the control terminal TCTRL takes a high level, the on/off circuit 21 makes the drive circuit 13 activate the switching control operation, and thus makes the output current lout flow through the white light-emitting diodes LED1 to LED6. On the other hand, when the brightness adjusting signal takes a low level, the on/off circuit 21 makes the drive circuit 13 deactivate the switching control operation, and thus lowers the output voltage Vout. This makes the average current flowing through the white light-emitting diodes LED1 to LED6 vary according to the duty of the brightness adjusting signal. The brightness of the white light-emitting diodes LED1 to LED6 is proportional to the average current flowing therethrough. This makes it possible to adjust the brightness of the white light-emitting diodes LED1 to LED6 by changing the duty of the brightness adjusting signal.
When the drive circuit 13 starts the switching control operation, the soft-start circuit 20 gradually changes the output duty of the drive circuit 13, and thereby makes the output voltage Vout rise gently. In other words, the soft-start circuit is a circuit that performs so-called soft-start operation. If the output voltage Vout does not rise gently, when the capacitor 5 is uncharged, an excessive charge current flows from the direct-current source 1 to charge it. The problem here is that when the direct-current source 1 is a battery such as a lithium-ion battery, such an excessive charge current puts an extra strain on the battery. Furthermore, this excessive charge current lowers the battery voltage, and thus makes it impossible to use the battery to its end voltage.
As shown in FIG. 11B, the soft-start circuit 20 consists of terminals T1 to T3, switches SW1 and SW2, a constant current source I1, a capacitance C1, and a P-channel MOSFET Q1 (hereinafter referred to as a Pch transistor). The terminal T1 is connected to the control terminals of the switches SW1 and SW2, and the terminal T2 is connected, via the switch SW1 and the constant current source I1, to the gate of the Pch transistor Q1, to one end of the capacitance C1, and to one end of the switch SW2. The other end of the capacitance C1, the other end of the switch SW2, and the drain of the Pch transistor Q1 are connected to the ground, and the source of the Pch transistor Q1 is connected to the terminal T3. Note that the terminal T1 is connected to the control terminal TCTRL, the terminal T2 is connected to the power supply terminal TVIN via the switch 25, and the terminal T3 is connected to the node at which the non-inverting input terminal of the PWM comparator 17 and the output terminal of the error amplifier 18 are connected together. The switch SW1 is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto. The switch SW2 is turned off when a high level signal is fed to its control terminal, and is turned on when a low level signal is fed thereto.
In the switching power supply circuit shown in FIG. 11A, every time a brightness adjusting signal VCTRL inputted to the control terminal TCTRL turns from a low level to a high level, the soft-start circuit 20 performs soft-start operation so as to cause the output voltage Vout to rise gently. This prevents the output voltage Vout from rising quickly to a predetermined voltage value V1. The problem here is that when the brightness adjusting signal VCTRL has a short cycle and thus remains at a high level only for a short period of time, it is impossible to apply to the white light-emitting diodes LED1 to LED6 a voltage required by them to emit light. This makes it impossible to perform desired brightness adjustment according to the duty of the brightness adjusting signal.
In view of the conventionally experienced inconveniences and disadvantages described above, the inventors of the present invention have devised a switching power supply circuit that activates/deactivates stepping-up operation according to an external signal and that can make the output voltage reach a target value even when a cycle with which the stepping-up operation is switched between an activated state and a deactivated state is short. Such a switching power supply circuit has been proposed, in the Japanese Patent Application filed as No. 2004-65427, by the applicant of the present invention.
The switching power supply circuit proposed in the above Patent Application differs from the switching power supply circuit shown in FIG. 11A only in that the soft-start circuit 20 is replaced with a soft-start circuit shown in FIG. 12 or a soft-start circuit shown in FIG. 13. Note that, in FIGS. 12 and 13, such members as are found also in FIG. 11B will be identified with common reference characters.
The soft-start circuit shown in FIG. 12 consists of terminals T1 to T3, a switch SW1, a constant current source I1, a capacitance C1, a Pch transistor Q1, a switch SW2, and a constant current source I2. The terminal T1 is connected to the control terminals of the switches SW1 and SW2, and the terminal T2 is connected, via the switch SW1 and the constant current source I1, to the gate of the Pch transistor Q1, to one end of the capacitance C1, and to one end of the switch SW2. The other end of the capacitance C1 and the drain of the Pch transistor Q1 are connected to the ground, the source of the Pch transistor Q1 is connected to the terminal T3, and the other end of the switch SW2 is connected to the ground via the constant current source I2. Note that the terminal T1 is connected to the control terminal TCTRL, the terminal T2 is connected to the power supply terminal TVIN, and the terminal T3 is connected to the node at which the non-inverting input terminal of the PWM comparator 17 and the output terminal of the error amplifier 18 are connected together. The switch SW1 is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto. The switch SW2 is turned off when a high level signal is fed to its control terminal, and is turned on when a low level signal is fed thereto.
When the input voltage Vin is fed to the terminal T2 and the brightness adjusting signal fed to the terminal T1 takes a high level, the switch SW1 is turned on and the switch SW2 is turned off. This makes the constant current source I1 output a constant current, and thereby charging of the capacitance C1 is started. After charging of the capacitance C1 is started, the gate potential of the Pch transistor Q1 gradually changes from a low potential to a high potential until the Pch transistor Q1 is switched from on to off. The gate potential ΔV1 of the Pch transistor Q1 is expressed by formula (1) below. It is to be noted that C1 represents the capacitance value of the capacitance C1, I1 represents the output current value of the constant current source I1, and Δt1 represents the charging time.ΔV1=I1·Δt1/C1  (1)
As the gate potential of the Pch transistor Q1 changes from a low potential to a high potential, the output potential of the error amplifier 18 gradually increases. This realizes soft-start operation. The rising speed of the output voltage Vout with soft-start operation can be easily set by adjusting the capacitance value C1 of the capacitance C1 and the output current value I1 of the constant current source I1.
On the other hand, when the brightness adjusting signal fed to the terminal T1 turns to a low level, the switch SW1 is turned off and the switch SW2 is turned on. This makes the constant current source I2 extract the charges accumulated in the capacitance C1. The charge extraction described above causes a potential drop ΔV2 across the gate of the Pch transistor Q1, which is expressed by formula (2) below. It is to be noted that C1 represents the capacitance value of the capacitance C1, I2 represents the output current value of the constant current source I2, and Δt2 represents the charge extraction time.ΔV2=I2·Δt2/C1  (2)
By reducing the potential drop ΔV2, it is possible to suppress extraction of the charges accumulated in the capacitance C1. Thus, it is necessary to make the ratio I1/I2 greater to suppress extraction of the charges accumulated in the capacitance C1. This makes it possible to suppress reduction in the output potential of the error amplifier 18.
When reduction in the output potential of the error amplifier 18 is suppressed, the soft-start circuit shown in FIG. 12 cannot perform soft-start operation. This allows the output voltage Vout to reach the target value (=V1) even when the brightness adjusting signal VCTRL remains at a high level only for a short period of time, as seen in FIG. 14 showing the waveforms of the brightness adjusting signal VCTRL fed from the outside of the stepping-up switching power supply device provided with the soft-start circuit shown FIG. 12 or FIG. 13, of the output voltage Vout, and of the input current Iin fed from the direct-current source 1. This makes it possible to make the LED1 to LED6 emit light. This makes it possible, even when the brightness adjusting signal has a short cycle and thus remains at a high level only for a short period of time, to perform a desired brightness adjustment according to the duty of the brightness adjusting signal. Note that the ratio I1/I2 described above is so set that the period required to complete discharging of the capacitance is made longer than a cycle with which the on/off circuit 21 switches the switching control operation of the drive circuit 13 between the activated state and the deactivated state.
In FIG. 14, assuming that the switching power supply circuit is started up at the time point when the brightness adjusting signal VCTRL turns from a low level to a high level for the first time, and, when the switching power supply circuit is started up, the output voltage Vout is 0 (V), and the output capacitor 5 is not charged at all.
When a brightness adjusting signal that turns to a low level when the power source switch provided between the direct-current source 1 and the switching power supply circuit is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit is inputted to the stepping-up switching power supply circuit provided with the soft-start circuit shown in FIG. 12, the charges accumulated in the capacitance C1 are extracted by the constant current source I2 when the power source switch is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit. In that case, the time when the power source switch is off, i.e. the charge extraction time Δt2 is long enough to allow the constant current source I2 to extract a sufficient amount of charges from the capacitance C1. This makes the gate potential of the Pch transistor Q1 equal to or nearly equal to the ground potential. This permits the soft-start circuit shown in FIG. 12 to perform soft-start operation when the power source switch is switched from off to on.
The soft-start circuit shown in FIG. 13 consists of terminals T1 to T3, a switch SW1, a constant current source I3, a capacitance C1, a Pch transistor Q1, and a constant current source I4. The terminal T1 is connected to the control terminal of the switch SW1, and the terminal T2 is connected, via the switch SW1 and the constant current source I3, to the gate of the Pch transistor Q1, to one end of the capacitance C1, and to one end of the constant current source I4. The other end of the capacitance C1, the drain of the Pch transistor Q1, and the other end of the constant current source I4 are connected to the ground, and the source of the Pch transistor Q1 is connected to the terminal T3. Note that the terminal T1 is connected to the control terminal TCTRL, the terminal T2 is connected to the power supply terminal TVIN, and the terminal T3 is connected to the node at which the non-inverting input terminal of the PWM comparator 17 and the output terminal of the error amplifier 18 are connected together. The switch SW1 is turned on when a high level signal is fed to its control terminal, and is turned off when a low level signal is fed thereto.
When the input voltage Vin is fed to the terminal T2 and the brightness adjusting signal fed to the terminal T1 takes a high level, the switch SW1 is turned on. This makes the constant current source I3 output a constant current. Of this constant current, a part is extracted by the constant current source I4, and the rest serves as a charge current for the capacitance C1. As the capacitance C1 is charged, the gate potential of the Pch transistor Q1 gradually increases from a low potential to a high potential until the Pch transistor Q1 is switched from on to off. The gate potential ΔV1 of the Pch transistor Q1 is expressed by formula (3) below. It is to be noted that C1 represents the capacitance value of the capacitance C1, I3 represents the output current value of the constant current source I3, I4 represents the output current value of the constant current source I4, and Δt1 represents the charging time.ΔV1=(I3−I4)·Δt1/C1  (3)
As the gate potential of the Pch transistor Q1 changes from a low potential to a high potential, the output potential of the error amplifier 18 gradually increases. This realizes soft-start operation. The rising speed of the output voltage Vout with soft-start operation can be easily set by adjusting the capacitance value C1 of the capacitance C1, the output current value I3 of the constant current source I3, and the output current value I4 of the constant current source I4. In the soft-start circuit shown in FIG. 12, when the gate potential of the Pch transistor Q1 is equal to or smaller than a predetermined value, there is no choice but to use a leakage current to extract the charges accumulated in the capacitance C1 due to the resistance of the switch SW2. On the other hand, in the soft-start circuit shown in FIG. 13, it is possible to reliably extract the charges accumulated in the capacitance C1 by using the output current of the constant current source I4.
On the other hand, when the brightness adjusting signal fed to the terminal T1 turns to a low level, the switch SW1 is turned to off. This makes the constant current source I4 extract the charges accumulated in the capacitance C1. The charge extraction described above causes a potential drop ΔV2 across the gate of the Pch transistor Q1, which is expressed by formula (4) below. It is to be noted that C1 represents the capacitance value of the capacitance C1, I4 represents the output current value of the constant current source I4, and Δt2 represents the charge extraction time.ΔV2=I4·Δt2/C1  (4)
By reducing the potential drop ΔV2, it is possible to suppress extraction of the charges accumulated in the capacitance C1. Thus, it is necessary to make the ratio I3/I4 greater to suppress extraction of the charges accumulated in the capacitance C1. This makes it possible to suppress reduction in the output potential of the error amplifier 18. Here, as far as charging of the capacitance C1 is concerned, the current extracted by the constant current source I4 is wasted. Thus, the ratio I3/I4 is preferably made greater than the ratio I1/I2 by making the output current value I4 of the constant current source I4 smaller.
When reduction in the output potential of the error amplifier 18 is suppressed, the soft-start circuit shown in FIG. 13 cannot perform soft-start operation. This allows the output voltage Vout to reach the target value (=V1) even when the brightness adjusting signal VCTRL remains at a high level only for a short period of time, as seen in FIG. 14 showing the waveforms of the brightness adjusting signal VCTRL fed from the outside of the stepping-up switching power supply device provided with the soft-start circuit shown FIG. 12 or FIG. 13, of the output voltage Vout, and of the input current Iin fed from the direct-current source 1. This makes it possible to make the LED1 to LED6 emit light. This makes it possible, even when the brightness adjusting signal has a short cycle and thus remains at a high level only for a short period of time, to perform a desired brightness adjustment according to the duty of the brightness adjusting signal. Note that the ratio I3/I4 described above is so set that the period required to complete discharging of the capacitance is made longer than a cycle with which the on/off circuit 21 switches the switching control operation of the drive circuit 13 between the activated state and the deactivated state.
When the power source switch provided between the direct-current source 1 and the switching power supply circuit is off, i.e. when the input voltage Vin is not fed to the switching power supply circuit, the charges accumulated in the capacitance C1 are extracted by the constant current source I4. In that case, the time when the power source switch is off, i.e. the charge extraction time Δt2 is long enough to allow the constant current source I4 to extract a sufficient amount of charges from the capacitance C1. This makes the gate potential of the Pch transistor Q1 equal to or nearly equal to the ground potential. This permits the soft-start circuit shown in FIG. 13 to perform soft-start operation when the power source switch is switched from off to on.
In order to produce ICs at lower cost, it is necessary to provide a single terminal with a plurality of capabilities and use a package with a small number of pins. Thus, in the stepping-up chopper regulator provided with the soft-start circuit shown in FIG. 12 or FIG. 13, the control terminal TCTRL is given capabilities of reducing the power consumption to nearly zero when the switching power supply circuit is off, of performing soft-start operation when the switching power supply circuit is switched from off to on, and of stopping soft-start operation when the brightness of the white LED is adjusted according to the duty of the brightness adjusting signal. However, power supply to all the circuits is stopped when the switching power supply circuit is off, because the power consumption is reduced to nearly zero when the switching power supply circuit is off. Therefore, it takes some time to start all the circuit portions after the switching power supply circuit is turned on. This is especially disadvantage because the feedback system does not operate properly until the circuit portion that serves as a power source for various amplifiers is started up.
When the constant voltage circuit 24 that converts the input voltage Vin fed from the direct-current source 1 into a predetermined voltage Vc has a configuration shown in FIG. 15, the constant voltage circuit 24 is not started up until a parasitic capacitance PC is fully charged. In a case where the Nch transistor 12 is turned on when the constant voltage circuit 24 is not started up, the Nch transistor 12 is kept turned on until the constant voltage circuit 24 is started up because the amplifiers (the PWM comparator 17, the error amplifier 18, and the like) are not controlled.
This does not cause any problem for the switching power supply circuit shown in FIG. 11A, because soft-start operation is performed when the switching power supply circuit is turned on. However, in the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13, when the Nch transistor 12 is kept turned on until the constant voltage circuit 24 is started up, a flow-through current flows through the Nch transistor 12 (see FIG. 16). In FIG. 16, VCTRL represents the brightness adjusting signal fed from the outside to the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13, Vout represents the output voltage of the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13, ISW represents the drain current of the Nch transistor 12 of the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13, and PI represents the flow-through current flowing through the Nch transistor 12 of the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13.
As described above, in the switching power supply circuit provided with the soft-start circuit shown in FIG. 12 or FIG. 13, when the brightness of the white LED is adjusted according to the duty of the brightness adjusting signal, excessive current flows through the Nch transistor 12 every time the switching power supply circuit is turned on. This makes the battery reach its end voltage (about 3.0V) earlier than expected. This disadvantageously restricts the length of the battery time with one charge.